(1) Field of the Invention
The present invention relates to an improved hydraulic load cell and a method for making same, and more particularly to a novel structure for a hydraulic load cell that is assembled by welding, thereby substantially eliminating mechanical fasteners. A self-centering column for transmitting the loading force to a load cell piston is also provided.
(2) Description of the Prior Art
Hydraulic load cells, which are generally used to weigh large or heavy loads, are known in the art. For example, U.S. Pat. Nos. 3,178,937 to Bradley; 3,145,795 to Tate; 3,089,519 to Bradley; 3,089,518 to Bradley; and 2,960,328 to Tate all disclose typical hydraulic load cells.
The typical hydraulic load cell consists of a piston adapted to move downwardly in response to a loading force into the interior of a closed cylinder. The piston includes a loading head and body, the loading head for attachment to an object to be weighed. Such downward movement acts upon fluid contained within a pressure chamber sealed by a diaphragm. The chamber communicates with a suitable pressure sensitive indicating device, such as a pressure transducer or a Bourdon tube mechanism, through a pressure transmitting passageway and indicator conduit. The indicating device may be calibrated to read directly in terms of pounds or grams of force or in any other desired unit.
The piston is typically supported axially within the cylinder by an annular stay plate and a bridge ring. Both the stay plate and the bridge ring are interposed between the piston and cylinder walls so that they will offset side thrusts on the piston without materially affecting its axial movement in response to a loading force. The stay plate and the bridge ring merely deflect upon movement of the piston and minimize inaccuracies stemming from off-center loading. They are essentially frictionless.
The cylinder is formed by bolting an open-ended substantially tubular wall element to a flat base with bolts. The bolted arrangement also clamps the diaphragm in place over the pressure chamber. The stay plate is fastened to the upper portion of the piston by a clamp consisting of an annular piston clamping ring bolted to the piston body by piston bolts. At its outer end, the stay plate is similarly clamped to the cylinder wall by an annular cylinder clamping ring and a series of cylinder bolts.
A rolling ball coupling, positioned between the loading head and the piston body, decreases the effect of side loading and reduces off-center loading. The coupling eliminates the moment that would be exerted on the piston by weight loads applied through a non horizontal load plate on the loading head and reduces the torque applied to the piston by a rotating load. The ball coupling also accommodates horizontal motion resulting from expansion and contraction caused by temperature changes of weighted objects resting thereon.
Typically the ball coupling consists of a hardened metal ball interposed between two hardened metal inserts. A rubber doughnut-shaped ring, preferably of neoprene rubber, positions the ball centrally within a cavity in the piston body and also acts as a shock absorber against lateral forces. Because such a ball coupling acting upon flat surfaces would permit lateral movement to occur, horizontal stay bars must be provided for restraining horizontal but permitting vertical motion.
If desired, the opposing surfaces of the metal inserts may be made with a concave surface to aid in centering the ball.
The coupling permits slight pivoting and some axial rotation of the loading head about the ball. The load is therefore still vertically transmitted to the piston body. Also the transmission of torque generated by the axial rotation to the piston body is minimized. The coupling tends to absorb transverse forces because the ball will roll and accommodate the displacement.
It is also known to provide a column having a self-centering tendency, where bearing surfaces on either end of the column are spherical and have geometric centers that lie along the center or longitudinal axis of the column but do not coincide, and the radii of the bearing surfaces are equal but are greater than one-half the length of the column, whereby a restoring force is created when the column is displaced laterally.
A flexible boot, preferably of synthetic rubber or the like, is suitably fastened to the cylinder wall and to the loading head by adjustable clamps to seal the interior of the cell from dirt, moisture, or other contaminants.